>I don't think we have the technology>to manipulate chromosomes at the level which would be necessary to change>the sex of an embryonic cell. But it would certainly be an interesting>experiment to do so and let the resulting male/female nearly-identical>twins breed.

No, no, that would be catastrophic - much worse than a brother-sister mating
The child would have a 1/4 chance of expressing any deleterious recessive
the original had masked by the normal gene, and for humans that's a lot.
Such children would often die, and would be virtually guaranteed major
genetic defects.

Cloning doesn't generate this problem as the chromosomes don't undergo
meosis (swap genes). So masked recessives stay that way. There will
be problems from accumulating mutations, but these should be minor for
a couple of generations.